Questions for assessing recent influential theory

Here is a list of questions that address the adequacy of theory to represent competitive interactions between species:

1. Does the model used include explicit resource dynamics (meaning separate dynamical equations describing temporal change in the consumer and resource populations)?

2.

a. What is the form of resource dynamics, and is more than one form explored?

b. How many resources are present, and how does that compare with the number of consumers present?

c. Do the resources interact directly with each other or via indirect pathways that do not include the consumers?

3. Does the model consider more than two competing species, and does it apply to systems with more species?

4. What are the characteristics of the functional and numerical responses of the consumers that are implied or assumed by the model?

5. Is the goal of the study to describe the interaction or only to predict whether the species will coexist?

These questions underlie my assessment of a set of highly cited articles listed in Table 4.1 below. I do not mean to argue that it is necessary that all the issues under­lying these questions be addressed in every study. This list of questions is certainly not comprehensive in terms of the processes that could be included, the functional forms that could be used, and the questions that could be asked. Given our current rudimentary understanding of the interaction known as competition, it is impossible to give a full accounting of the requirements for a predictive theory. However, at least some of the required features of a better competition theory seem clear, and are listed below:

(1) A priori, it would seem almost impossible that resource properties would not affect competition, and many supportive examples are given throughout this book. Thus, a general theory regarding competition should incorporate resources, and be capable of dealing with different effects of consumed resources on the per capita rate of increase of the consumer.

(2) The 2-competitor case represents the minimum number possible for interspe­cific competition, and it is usually less than the number of species that share resources in natural communities (Yodzis 1988). Having equal numbers of con­sumer and resource species is also unlikely; given the roles of space and time in defining resources there are likely to be many more distinct resources than con­sumer species. Furthermore, equal numbers of consumers and resources imply that exclusion of at least one consumer is bound to follow upon elimination of one of the resources, provided the system has a stable equilibrium. Having equal numbers of consumer species and resource types also implies that, for a large class of models, a sufficient increase in the abundance of one competitor will eventu­ally cause the extinction of all others, something that is not guaranteed when at least some consumers have exclusive use of one or more resources.

(3) An adequate theory should be able to predict how changed environmental con­ditions affecting one consumer alter the abundances of others, rather than just determining whether or not they can coexist. Most changes in abundance that are caused by competitors do not involve extinction, but do involve different effects on the resource-related traits of any given consumer. A useful description of the dynamics allows such changes to be understood and their effects on consumer abundances to be predicted.

(4) Finally, a general theory of interspecific competition should provide a frame­work that can be used to understand the co-occurring interaction of intraspecific competition and to understand the evolution of traits that determine both intra- and interspecific competition. These two areas of investigation involve the same types of resource-mediated effects as interspecific competition. The closely relat­ed interaction of ‘apparent competition', defined by effects transmitted by shared predators, requires the same focus on the dynamics of the transmitting enti­ty. Probably for historical reasons, theory on apparent competition has always included this feature.

Each of the above four requirements had been recognized by some investigators by the beginning of the 1980s, and at least the first two seemed to be widely accepted at that time. However, they do not seem to be a major concern for most of the recent articles on competition listed below.

4.3